Vinylidene fluoride (VDF) copolymers comprising recurring units derived from hydrophilic (meth)acrylic monomers (e.g. acrylic acid) are well known in the art.
Such copolymers have been developed aiming at adding to the mechanical properties and chemical inertness of PVDF suitable adhesion towards metals, e.g. aluminium or copper, or hydrophilic properties.
Often, these materials are produced by surface functionalization of pre-formed PVDF materials, so as to obtain grafted copolymers; nevertheless such grafted copolymers suffer from the drawback that as grafting is generally initiated via radical route, by providing free radicals generation on the fluorinated backbone: uncontrolled radical side reactions occur, yielding poorly soluble, generally cross-linked materials, and/or generating chains breaks, so that the final material possess impaired mechanical properties.
Linear copolymers are thus generally preferred when a precise control of the molecular structure is required.
U.S. Pat. No. 4,861,851 (JAPAN SYNTHETIC RUBBER CO LTD (JP)) 29 Aug. 1989 discloses substantially random linear vinylidene fluoride acrylic acid ester copolymers comprising, inter alia, 15 to 95% wt of at least one unit selected from alkyl acrylate and alkoxy-substituted alkyl acrylate. Said copolymers can be produced by copolymerizing suitable monomers in emulsion, suspension, solution or bulk polymerization, emulsion being preferred.
WO 01/57095 9 Aug. 2001 discloses linear fluoropolymers comprising fluorocarbon and hydrocarbon repeat units, fluorocarbon units being derived from tetrafluoroethylene, chlorotrifluoroethylene, trifluoroethylene, vinyl fluoride or vinylidene fluoride, the hydrocarbon units being derived from vinyl acetate, vinyl alcohol, acrylic acid, acrylated, methacrylates. Said materials are prepared by polymerization in a medium comprising supercritical carbon dioxide. WO 01/57095 9 Aug. 2001 teaches that acrylic acid monomers have intrinsic reactivity and tend to reacts with themselves rather than with the fluorocarbon monomer, also due to the lack of physical affinity for domains of the other monomer. “Blocky” structures are generally obtained as a function of polymerization conditions. In CO2 supercritical polymerization, the choice of selected surfactants can improve the homogeneity of the hydrocarbon monomers' distribution.
U.S. Pat. No. 5,880,204 (ALLIED SIGNAL INC (US)) 9 Mar. 1999 discloses room temperature coalescable aqueous fluoropolymer dispersions comprising fluoropolymer particles having a core-shell structure, comprising a core composed predominantly of a first semi-crystalline fluorocopolymer, and a shell composed predominantly of a second amorphous fluorocopolymer obtained by reacting, in the presence of said first copolymer, at least one cure site provider (e.g. acrylic acid), and at least two fluorinated comonomer units. Said second polymerization step yielding the amorphous terpolymer can be carried out by continuous addition of the cure site provider.
Copolymerizing vinylidene fluoride and hydrophilic (meth)acrylic monomer remains indeed an uneasy task, due to the very different reactivity of such copolymers and their intrinsic incompatibility.
Thus, while random distribution is targeted, blocky-type structures are obtained. This uneven distribution of the hydrophilic (meth)acrylic monomer units dramatically affects the thermal stability of the copolymer itself.
This problem has been found particularly critical in copolymers comprising lower amount of hydrophilic (meth)acrylic monomers, especially of acrylic acid, wherein the formation of blocky domains of concatenated acrylic acid substantially depletes the resulting copolymers of the targeted properties, so that no modification in adhesiveness is obtained. Increasing level of acrylic acid incorporation on the contrary dramatically impacts the thermal stability of the fluoromaterial.
There is thus still a need on the art for vinylidene fluoride linear copolymers comprising recurring units derived from hydrophilic (meth)acrylic monomers, possessing a substantially random distribution of the recurring units derived from hydrophilic (meth)acrylic monomers throughout the whole vinylidene fluoride backbone, and thus exhibiting improved thermal stability.
Also, there is a need in the art for a polymerization process enabling easy manufacture of the copolymers as above described.